This invention relates generally to the formation of positive relief images in layers of radiation degradable polymers and more specifically to a process for forming positive resist masks using haloalkyl methacrylate copolymers.
The formation of positive resist masks from layers of radiation degradable methacrylate polymers is described, for example, by Haller et al in U.S. Pat. No. 3,535,137; by Gipstein et al in U.S. Pat. No. 3,779,806; in copending application Ser. No. 545,063 filed Jan. 29, 1975, now U.S. Pat. No. 4,011,351, and assigned to the assignee of this application entitled "Preparation of Resist Image With Methacrylate Polymers"; by R. A. Harris "Polymethyl Methacrylate as an Electron Sensitive Resist" Journal of Electrochemical Society, Vol. 120, No. 2, pages 270-274, February 1973; and by E. D. Roberts "A modified Methacrylate Positive Electron Resist" Applied Polymer Symposium, No. 23, pages 87-98 (1974). The masks are useful in the fabrication of integrated circuits, printing plates and the like.
Generally, the resist masks are prepared by coating a layer of the radiation degradable polymer on a substrate and subjecting the layer to patternwise exposure to high energy radiation such as x-rays or electrons. The substrate may be pre-treated with an adhesion promotor before coating the resist layer. The irradiated portions of the resist are degraded and become more soluble. A developer is then used to remove the irradiated portions of the layer. The substrate can then be subjected to an additive or substractive process such as metallization, ion implantation, or etching with the remaining portions of the resist layer acting to protect the substrate from the processing. When the exposure is made with a focused, scanning electron beam, the exposure step is a lengthy process with the time required being largely dependent upon the exposure energy which is required to degrade the polymer to a sufficient extent so that a satisfactory pattern can be reproducably developed in a reasonable time without excessive loss or swelling of the unexposed resist layer.
Haller et al, U.S. Pat. No. 3,535,137 describes the use of methacrylate polymers containing a quaternary carbon in the polymer backbone such as polymethyl methacrylate, and copolymers of methyl methacrylate with, for example, 2-hydroxyethyl methacrylate. Gipstein et al, U.S. Pat. No. 3,779,806 discloses the use of certain polymers of t-butyl methacrylate for forming resist masks using electron beam exposure. Application Ser. No. 545,063, now U.S. Pat. No. 4,011,351, discloses the use of non-cross-linked films including copolymers of alkyl methacrylates and ethylenically unsaturated monomers containing a halogen substituted substituent in amounts of 1 to 50 mole percent where the halogen substituent is attached either to a carbon of the polymer chain or to a methyl group attached to a carbon of the polymer chain or post halogenated alkyl methacrylate polymers. The E. D. Roberts article discloses the use of cross-linked films which are prepared by heating mixtures of a copolymer of methyl methacrylate and methacrylic acid with a copolymer of methyl methacrylate and methacryloyl chloride to form anhydride linkages by reaction of the acid chloride and acid groups.
The processes described in U.S. Pat. Nos. 3,535,137; 3,779,806; and application Ser. No. 545,063, now U.S. Pat. No. 4,011,351, involve main chain degradation and/or the splitting off of the ester moiety in order to lower the molecular weight of the polymer in the exposed areas and thereby increase the solubility of the exposed areas. Roberts, on the other hand, introduces anhydride cross-links which initially insolublize the polymer layer upon prebaking by not only increasing the molecular weight but by changing the nature of the functional groups. These cross-links are then broken during exposure to make the exposed portions soluble. Roberts requires mixtures of two different copolymers and his reported data indicates that only a portion of the acid chloride and acid groups react to form cross-links. Accordingly, the nature of any particular unexposed resist film would be unpredictable. Additionally, the unexposed film is not soluble in ordinary solvents so that stripping the layer after it has fulfilled its purpose involves chemically breaking down the material such as by reaction with a concentrated base to hydrolyze the cross-links before the film can be removed. Such treatments can have an adverse affect on certain substrate layers which are involved in integrated circuit formation.
A process has now been found which employs a different group of alkyl methacrylate copolymers which, upon prebaking, cross-link in a predictable fashion in that complete reaction of the cross-linking moiety is approached. A more radiation sensitive higher molecular weight polymer structure is produced which requires only a single copolymer to form the resist layer without any change in functionality occurring by the cross-linking reaction. Therefore, the unexposed films can be easily stripped by solvents without the need to chemically attack the polymer.